Jason Segel is said to be over the moon with girlfriend Michelle Williams, and from the looks of his day out with her daughter, Matilda Ledger, he's ready to take on dad-like duties, too!

In Brooklyn, NYC on Wednesday, six-year-old Maltida, Williams' daughter with her late ex-fiance Heath Ledger, seemed very comfortable and at ease with The Five Year Engagement actor and even let the actor hold her tiny hand.

Adds another source, "Jason is obsessed with Michelle and wants to be a father figure to Matilda," says a source close to the actor. "He's been doing all sorts of 'dad stuff' with her, like drawing and playing music. They've been spending time together whenever possible."

And while it's clear that Segel, 32, has earned Matilda's seal of approval since taking his romance with her mom, 31, public in March 2012, his friends weren't so sure how he'd fare at first. "I never thought Jason would ever date someone with a kid," a Segel confidante told Us. "But he's taking everything in stride. He's ready to be a grown-up."